He then appeared to breach protocol by disclosing that he had received an unusual briefing from Sir John Sawers, the chief of the Secret Intelligence Service (MI6), on the situation in Syria. An adviser boasted to The Daily Telegraph that Mr Romney had also previously met the head of the Security Service (MI5).

Mr Romney told NBC News he saw “a few things that were disconcerting” about London’s preparations. “The stories about the private security firm not having enough people, supposed strike of immigration and customs officials, that obviously is not something which is encouraging,” he said.

His remark prompted a sharp rejoinder from David Cameron. “We are holding an Olympic Games in one of the busiest, most active, bustling cities anywhere in the world,” said the Prime Minister. “Of course it’s easier if you hold an Olympic Games in the middle of nowhere.”

This was widely taken in the US to be a reference to Utah, the sparsely populated western state where Mr Romney was chief executive of the Salt Lake City Winter Olympics in 2002. “Romney Olympics comments rile Brits,” CNN told its viewers throughout the afternoon.